Today's energy shortages make it increasingly necessary to store energy which becomes available during periods of relatively low energy demand for use during peak demand periods. For example, solar energy is readily available during relatively low daytime demand periods, but is frequently unavailable during the peak evening hour demand periods.
It has been suggested to store excess energy with inertial energy storage wheels or rotors. Such energy storage requires that excess energy, normally electrical power, is used to drive an electrical motor-generator to spin the rotor at often very high rates of rotation. To recover the energy, the motor-generator is operated in its generator mode to generate electricity while correspondingly decreasing the rotor's rate of rotation, thereby converting the rotor's inertial energy into electrical power. To store a meaningful amount of energy, the rotors have to be spun at rates as high as 20,000 rpm and more, depending on their diameter. This severely stresses the rotor and requires that it be specially constructed so that it can withstand the centrifugal forces generated by such high rates of rotation.
It is known that the stress to which a rotating ring is subjected comprises both hoop or circumferential stresses, which subject the ring material to tension, and radial stresses which subject the material to translaminar tension. In the radial direction the tensile stresses are carried by the matrix material only which is relatively weak. Since the radial tensile stress depends upon the ring thickness to radius ratio, the ring must be relatively thin to maintain the stresses within the limits of the matrix material.
To achieve the necessary high energy storage densities requires materials with a high strength-to-weight ratio. The materials with the highest strength-to-weight ratios currently are fiber materials such as those used for the reinforcement of plastic composites. The fiber composites, therefore, offer the potential of very high energy storage densities. Problems exist, however, due to the orthotropic properties of the composites. They possess very high strength in the direction of the fibers, that is in the circumferential direction, and very little strength in the transverse direction, that is, in a radial direction. Thus, fiber composite materials can withstand only very limited radial forces.
A theoretical ring with no radial thickness would not be subjected to any radial stress, but to hoop stresses only. Thus, to limit the radial stresses in such rings to acceptable values, their radial thickness must be relatively small. Accordingly, it has been suggested to construct inertial energy storage rotors by combining a plurality of relatively thin, concentric rings into one rotor. The rings are mounted to a concentric hub, which in turn rotates about a vertical axis. The rings are interconnected by resilient, e.g., elastomeric spacers disposed between each pair of adjacent inner and outer rings, U.S. Pat. Nos. 3,683,216 and 3,741,034 generally describe the construction of inertial energy storage wheels constructed of a plurality of concentric rotor rings carried by a common hub. Elastomeric spacer rings connect each inner ring to its adjacent outer ring.
During operation of the rotor wheel the rings dilate, that is, they expand by differing amounts which are directly related to their mean diameters. Consequently, the inter-ring connection as well as the connection of the innermost ring to the rotating hub must freely accommodate the relative inter-ring dilations as well as the dilation of the innermost ring with respect to the hub. Failure to do so would subject the rings to severe stresses which, as a practical matter, cannot be withstood by today's materials if the rotor wheel is to be spun at a meaningful, that is, a very high rate of rotation.
One method for mounting the innermost ring to the hub and of interconnecting the concentric rings is to provide radially oriented bars which snugly engage corresponding holes in the rings. Such bars center the rings with respect to each other and when the bars snugly engage corresponding holes in the ring they substantially immovably interconnect the rings. When the wheel rotates the radially oriented bars permit the rings to dilate and to move in a radial direction relative to the bars. The earlier mentioned danger of locking one ring to the other and of thereby losing the advantages obtained by constructing the rotor wheel of multiple, relatively thin rings is thereby prevented.
A drawback of such a construction, however, is the fact that as the rotor rings dilate they expand circumferentially. The width of the bar receiving holes expands proportionally. The initial, at rest snug fit between the bars and the holes is lost and some play between them develops. Although the play might be relatively small, at the high rates of rotation under consideration here, such play can prove fatal and in any event introduces a vibrational instability which, if not damaging to the rotor wheel itself, can lead to the premature failure of the bearings for the wheel and of associated equipment.